A pump stroke control system for a windmill is described and claimed in U.S. Pat. No. 4,392,785 to Don E. Avery, the disclosure of which and the description of the background references of which are incorporated herein by reference.
While the system described in U.S. Pat. No. 4,392,785 is completely suitable for use with windmills, a need persists for other control systems which may be as suitable and which may be particularly suited for high speed, low surface area windmills.
Low surface area windmills which are characterized, for example, by long aircraft-like propellers with diameters of 23 feet or more have many advantages. One obvious advantage of a high speed rotors is the greater amount of power which is available from it. Another advantage is a dramatically lighter rotor. Each of the three work blades in one example weighs about 21 pounds. With the hub included, the estimated weight of a 23 foot diameter rotor is about 100 pounds. A galvanized sheet metal multivane rotor would weigh over 2,000 pounds. In storm conditions there is much less area to be impacted making lighter supporting structure possible for low surface area, high speed rotors. In addition, the light rotor has a much smaller mass moment of inertia which means that it will respond better to gusty winds and pump more water due to the "wind speed cubed" factor in power output. Low surface area or low solidity rotors have low output torques at low speeds. The special characteristics of a low solidity, high speed windmill provides a requirement for a special output control system.
"Wind speed cubed" is a phrase well known to those familiar with and skilled in the art and simply means that the available power from the wind increases in proportion to the wind speed cubed (w.s..sup.3). This can be easily proved using the basic equation for kinetic energy which is KE=1/2 MV.sup.2. The wind force, when blowing perpendicular to a flat plate, is proportional to the wind speed squared.
For a more detailed explanation, reference is made to the inventor's prior U.S. Pat. No. 4,392,785. Wind blowing against flat plate 18 stretches spring 48 and causes point 72 to move in proportion to the wind speed squared. The location of point 72 fixes the stroke length and therefore the pump "size" and torque on the rotor shaft. If the spring constant of spring 48 is of the correct value, the windmill will operate at or near the tip speed ratio (TSR) for maximum output and the machine will speed up or slow down in direct proportion to the wind speed, thus causing the output to vary in proportion to the windspeed cubed.